The Detroit-Diesel 6-71 engines were all sourced from the Detroit Diesel Plant, in Detroit!
The only GMC engines built in Walkerville Plant were 248s, 1940-41. I have never seen any paperwork suggesting that 270 units were ever even thought of being built in Canada, though there is a hint that 248s were to be offered for CMPs in late 1939.
General Motors Ltd, Cleckheaton, Yorkshire figures for A vehicle engine rebuilds:
1943
“A” ENGINES [ARMOURED VEHICLES]:
VALENTINE ['G.M.C.'] 100
GRAY 2
QUAD 72
TOTAL 74
1944 “A” ENGINES [ARMOURED VEHICLES]:
VALENTINE ['G.M.C.'] 55
SHERMAN TWIN 4
TOTAL 59
QUAD 777
GRAY 190
TOTAL 967
1945 “A” ENGINES [ARMOURED VEHICLES]:
SHERMAN 4
G.M. DIESEL 32
TOTAL 36
QUAD 652
GRAY 207
TOTAL 859